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Item 8: Prevent exceptions from leaving destructors

Concept​

Destructors should never emit exceptions. If a destructor calls a function that might throw, the destructor should catch any exceptions and either swallow them or terminate the program. This is because if an exception is already active (stack unwinding) and a destructor emits a second exception, C++ calls std::terminate. If clients need to react to an exception thrown during an operation, the class should provide a regular (non-destructor) function that performs the operation.

Code Example​

class DBConn {
public:
    // Provide a way for clients to handle errors
    void close() {
        db.close();
        closed = true;
    }

    ~DBConn() {
        if (!closed) {
            try {
                db.close();  // last-ditch attempt
            } catch (...) {
                // Log the failure
                // Option 1: std::abort() if we can't continue
                // Option 2: swallow the exception (document this!)
            }
        }
    }

private:
    DBConnection db;
    bool closed = false;
};

// Client code can handle errors explicitly:
DBConn dbc(DBConnection::create());
dbc.close();  // Client gets a chance to react to errors
// Destructor serves only as a backup

Full source code

Things to Remember​

  • Destructors should never emit exceptions. If functions called in a destructor may throw, the destructor should catch any exceptions, then swallow them or terminate the program.
  • If class clients need to be able to react to exceptions thrown during an operation, the class should provide a regular (non-destructor) function that performs the operation.
  • Item 7 — Virtual destructors in polymorphic base classes
  • Item 29 — Exception-safe code and the nothrow guarantee